Conventional soda-lime-silica glass is a rigid amorphous solid that is used extensively to manufacture a variety of hollow glass articles including containers such as bottles and jars. Soda-lime-silica glass comprises a disordered and spatially crosslinked ternary oxide network of Na2O—CaO—SiO2, in which the molar ratio of Na2O:CaO:SiO2 is approximately 1:1:6, and may also include other optional oxide and non-oxide materials, which may be referred to as secondary additives, that act as colorants, decolorants, redox agents, or other agents that affect the properties the final glass. Some examples of these optional oxide and non-oxide materials include Al2O3, MgO, Li2O, K2O, Fe2O3, Cr2O3, MnO2, Co3O4, TiO2, SO3, and selenium. While the exact composition of the soda-lime-silica glass may be tailored to its particular end-use application by the inclusion of secondary additives, the Na2O—CaO—SiO2 ternary oxide network with its approximately 1:1:6 molar ratio is subject only to minor variances that fall usually within acceptable manufacturing tolerances.
Soda-lime-silica glass containers are typically produced by a melt processing procedure. Generally, during melt processing, a feedstock batch that includes virgin raw materials and optional recycled glass (i.e., cullet) is first melted in a continuous melting furnace at temperatures in excess of 1400° C. The resultant glass melt is homogenized and refined—usually downstream of the melting zone of the furnace—to achieve chemical and thermal consistency and to remove bubbles and inclusions. Glass containers are then fabricated from the homogenized and refined glass melt. For example, in a standard container-forming process, the glass melt is cooled in a forehearth channel to around 1150° C. and then distributed as individual gobs of molten glass to individual sections of an individual section forming machine by way of a gob delivery system. The glass gobs are formed into containers by a press-and-blow, a blow-and-blow, or some other shaping technique, typically at a temperature in excess of 900° C., followed by cooling of the containers to preserve their shape. The manufactured glass containers are then reheated and cooled at a controlled rate in an annealing lehr to remove internal stress points. Any of a variety of coatings may be applied to the surface of the glass container either before (hot-end coatings) or after (cold-end coatings) annealing.
The inclusion of Na2O and CaO in the chemistry of soda-lime-silica glass renders the commercial manufacture of glass containers more practical and less energy intensive while still yielding acceptable glass properties. The Na2O component functions as as a fluxing agent that reduces the melting, softening, and glass transition temperatures of the glass, as compared to pure silica glass, and the CaO component functions as a stabilizer that improves certain physical and chemical properties of the glass including its hardness and chemical resistance (especially with respect to water). Another oxide material, Al2O3, is commonly used in the glass container manufacturing industry to improve the chemical durability of the glass. But the use of Na2O, CaO, and other oxide materials along with the primary network former, SiO2, has to be balanced against the susceptibility to devitrification, or the spontaneous growth of crystals such as devitrite (Na2Ca3Si6O16) on the glass surface, since the dilution of SiO2 with network modifiers confers mobility within the glass oxide network, thus making it easier for molecular network chains to rearrange themselves into crystal structures.
Devitrification is generally undesirable during the manufacture of soda-lime-silica glass containers because it reduces the transparency and mechanical strength of the glass. Devitrification may occur in soda-lime-silica glass when the glass is held in a viscous supercooled liquid state at a temperature between its glass transition and liquidus temperatures for too long. And, as previously noted, the inclusion of network modifiers in the soda-lime-silica glass chemistry, in particular CaO, increases the susceptibility of the glass to devitrification by increasing the liquidus temperature of the glass and enhancing network chain mobility. The 1:1:6 molar ratio of Na2O:CaO:SiO2 in soda-lime-silica glass strikes the appropriate balance between energy consumption, the physical and chemical properties of the glass, and the ability to cool the containers relatively quickly through a viscous supercooled liquid state to a temperature below the glass transition temperature while avoiding devitrification.
Glass-ceramics are a different class of materials than amorphous glasses such as conventional soda-lime-silica glass. Unlike soda-lime-silica glass, in which devitrification is purposefully avoided, glass ceramics are formed by crystallizing or ceramizing a parent glass in a controlled manner to form a crystalline phase distributed within an amorphous residual glass phase. More specifically, in standard practices, a parent glass having a chemistry tailored to glass-ceramic processing is formed, usually by melt processing, and then heat-treated in a multi-step procedure to induce bulk internal nucleation followed by crystal growth. The bulk nucleation stage of the heat-treatment procedure induces nuclei seed formation homogeneously throughout the bulk of the parent glass, and the subsequent crystal growth stage, which may be conducted at a higher temperature, grows crystals from and around those seeds. As such, the crystals in glass-ceramics are homogeneously distributed within the amorphous residual glass phase, as opposed to being formed and concentrated on the glass surface as a result of the spontaneous and unwanted nucleation that typifies devitrification.
A wide variety of parent glass chemistries that are conducive to glass-ceramic manufacture are known. Some fairly common parent glass compositions that have gained widespread applications are simple silicates such as the Li2O—SiO2 system and aluminosilicates such as the Li2O—Al2O3—SiO2, MgO—Al2O3—SiO2, and ZnO—Al2O3—SiO2 systems. These and other parent glass compositions usually include nucleation agents that promote bulk nucleation via the formation of nuclei seeds throughout the parent glass. Examples of suitable nucleation agents include metals such as gold, silver, platinum, palladium, and titanium, and nonmetals such as fluorides, ZrO2, TiO2, P2O5, Cr2O3, and Fe2O3. As a result of the formation of a crystalline phase comprised of well-distributed fine-grain crystals, glass-ceramics tend to have higher strength, toughness, chemical durability, and electrical resistance than their noncrystallized parent glass, and also exhibit a relatively low coefficient of thermal expansion, which provides them with excellent thermal shock resistance.
Due to their unique and customizable properties, glass-ceramics have found a wide variety of applications including aerospace and military products, cookware, satellite and telescope optics, dental restorations, and as bioactive materials. The glass container manufacturing industry may also benefit from identifying glass-ceramics that can meet its needs for various types of standard and specialty containers. It has been determined that a glass-ceramic based primarily on the same Na2O—CaO—SiO2 ternary oxide system as conventional soda-lime-silica glass-albeit one in which the chemistry is more conducive to controlled crystallization-could potentially be a welcome addition to the glass manufacturing art. Techniques for manufacturing such a glass-ceramic have also been identified that consume less energy than customary practices of glass-ceramic manufacturing in which a parent glass is first produced at relatively high temperatures through melt processing.